For storage devices and the like, known examples of techniques for creating a copy of data include snapshot and mirroring techniques. In snapshot, a storage device logically creates a copy of a disk image instantly, and physically copies updated regions at a time when disk access is performed. This copying method is called “copy-on-write”. In snapshot, sometimes the storage device copies the entire data image in the background simultaneously with execution of the copy-on-write.
Mirroring is a technique in which updates made to a copy source volume are continuously transferred to a copy destination volume so that the contents of the copy source volume and the contents of the copy destination volume remain substantially the same. In mirroring, the storage device halts transfer from the copy source with a detach command (detach in duplexing) and then creates a copy of the copy source volume, which existed at the time when that command was issued, in the copy destination volume.
In order to support these techniques, sometimes a data image is divided into regular sections and whether or not copying has been completed is recorded for each of the sections. Data indicating whether or not copying has been completed in such a way is called a bitmap.
A copy creation device that performs copy creation processing using a bitmap is known. This copy creation device sets up a bitmap indicating whether copying to a copy destination volume has been performed or not for every predetermined region of the data of the copy source volume.
Japanese Laid-open Patent Publication No. 2008-225699 discloses a related technique.
However, in the above-described copy creation device, a bitmap has to be set up for each predetermined region of data of the copy source volume and a lot of resources may be used to set up such bitmaps.